Solid and Liquid States of Lactose

Lactose in dairy systems can exist in various crystalline and non-crystalline forms. These forms affect lactose behaviour, particularly in processing and storage of low-water dairy foods. Crystalline -lactose monohydrate and anhydrous -lactose are well-known solid forms of lactose, which are relatively poorly soluble in water. Its occurrence in two anomeric forms, and -lactose, makes its solubility a complex function of temperature. -Lactose has low solubility in water at room temperature, but mutarotation to equilibrium quantities of the and -forms increases the overall solubility of lactose which increases rapidly with increasing temperature, with a more rapid increase in the solubility of -lactose. Liquid dairy systems contain dissolved lactose in a complex chemical environment and lactose is likely to exist in a composition-, temperatureand process-dependent / -ratio. On rapid removal of solvent water from dairy liquids on dehydration or freezing, lactose molecules retain their solution structure and, therefore, amorphous, non-crystalline solid forms of lactose are typical of dairy powders and frozen dairy desserts (Roos, 1995; Hartel, 2001). Amorphous lactose in dairy solids may often exist in a glassy, solid state or in a syrup-like, super-cooled liquid state. The apparent glass-like solid state results from a very high viscosity exceeding 10 Pa s (White and Cakebread, 1966). The state transition of amorphous solidand liquid-like states occurs over a second-order-type state transition known as the glass transition (White and Cakebread, 1966), as described in Figure 2.1. The glass transition involves no latent heat but it can be observed from changes in heat capacity, thermal expansion coefficient, dielectric properties, various mechanical and flow properties and molecular mobility (White